Numerous attempts have been made to control the growth environment of crop plants so as to improve yield and control the timing of flowering. These include providing shelter from wind and heavy rain, as for example by errection of a "greenhouse", or by applying various chemicals which control growth by simulating the effects of the natural plant hormones. Inevitably there is a trade off, in the case of the greenhouse, scorching becomes a problem due to light focussing and the overheating created by the "greenhouse effect" of trapping radiant energy, whereas in the case of chemical application there are attendent problems associated with residues both in the food crop and in the soil.
In order to understand fully the significance of the current invention it is first necessary to review the constitution of terrestrial sunlight and to realize how plants exploit this radiation.
Terrestrial sunlight has been considered to consist of shortwave ultraviolet light (UV-B, 290-320 nm), long-wave ultra-violet light (UV-A, 320-400 nm), visible light (400-700 nm) and infra-red light (700-2500 nm). On a clear day at sea level, the distribution of solar radiation is about 1-2% UV, 42% visible and 57% infra-red. It is now generally recognized that the UV-B radiation is dangerous to all living cells leading to various genotoxic disorders by mutation and will cause, at high enough exposure levels, plant and animal cell death. UV-A on the other hand is apparently harmless to animal tissues at natural radiation levels, but green plants have utilized the hormone-destroying effect of UV-A light to control or regulate their growth, while protecting themselves as best they can from the dangerous UV-B rays with the pigment chlorophyll. It has been known since Darwin's time that plants produce a growth stimulating substance (hormone or auxin) near their growth tips which diffuses downwards promoting further growth and that if completely shaded, plants will grow, for a limited period, at about three times their illuminated rate (e.g. "forcing" of hyacynthe or rhubarb shoots) and that the bending of plants towards light is caused by the protection of the auxin from light denaturation on the darkside of plants grown on say a window sill. The specific wave-lengths of light which denature auxins lie in the UV-range, 290-340 nm and the photolysis of such auxins can occur even at quite meager light intensities. Prevention of photolysis would render the plants continuously sensitive to their own growth stimulation. In a similar manner, the induction of flower formation and development has been determined to be influenced by the presence and the intensity of natural red light and consequently flowering may be encouraged or delayed by manipulating the filtered environment so as to provide or absorb those wavelengths responsible. It is the purpose of this invention to provide such an environment or environments.